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30 Principle 2 Soil Preparation Grant E. Cardon, Ron Patterson, and Rick Heflebower Introduction Fundamental to water-efficient management in landscapes and gardens is an understanding of how soils interact with water and how various soil properties affect the infiltration, movement, and retention of water. In this section, we briefly introduce soils and soil properties, emphasizing those important to water relations. We also discuss manipulation of those properties to promote or facilitate good plant/water relations. Any given soil volume comprises four main components: unconsolidated mineral matter, organic matter, air, and water. The typical relative proportions of the components are shown in Figure 2-1. Soil Mineral Matter The mineral matter of the soil refers to the content of sand, silt, and clay. The proportions of these particle types give a soil its particular texture. Sand, silt, and clay are size classes of mineral particles, each imparting a characteristic influence on the soil’s physical, chemical, and hydraulic properties. The relative particle sizes that make up each class are shown in Figure 2-2. Individual sand particles are visible to the naked eye and provide the coarse, gritty feel of a soil. Silt particles are at the lower margin of the size class of sand Soil Preparation   31 Figure 2-1. Typical ranges for the four components in a volume of intermountain western soil. Figure 2-2. Relative size classes and textural character of soil particles. Composition of a Loam Soil Mineral Fraction (sand, silt, clay) = 50% Water = 20-30% Air = 20-30% Organics = 5% and are nearly one thousand times smaller than the largest sand particles. Both sand and silt particles are roughly spherical to blocky in shape. Clay particles, on the other hand, are very small, platelike particles crystallized from the weathering products of the soil’s primary minerals, or washed or blown onto a soil from other areas. Clay particles are from one thousand to many thousand times smaller than sand particles. The platy shape of these clay particles means that they have an enormous amount of surface area per unit volume, which provides habitat for soil microbes, as well as reactive sites for chemical reactions and water adsorption . One gram of clay can have as much as 1,000 square meters of reactive surface area. Therefore, clay exerts some of the strongest influence on a soil’s capacity for water retention. The relative proportion of sand, silt, and clay in the mineral matter of a soil is termed its soil texture. Soils having similar characteristics based on their texture are grouped together and named for convenience as shown in Figure 2-3. This is known as the soil textural triangle. Textural group names provide a ready reference to the proportion of sand, silt, and clay in a soil, as well as its suite of physical, chemical, and hydraulic properties. For instance, if one refers to a sandy loam and a silty clay, it is immediately understood that the two soils are widely divergent in their character because of their distance from one another on the soil textural triangle. The sandy loam soil has a highly gritty feel, larger particles and therefore larger pores, and much less reactive surface area than the silty clay. On the other hand, the silty clay is fairly sticky when wet and has a high water retention capacity, but will not allow for easy water infiltration due its much smaller pores compared to the sandy loam. Note the strong influence of clay content on the characteristics of soils. Above about 40% clay content (the upper third of the textural triangle), soils overwhelmingly take on the character of the clay particles and form a large single group. Soils in the loam category and in the immediately adjacent loam-modified categories have the optimal combination of large and small soil particles, large and small pores, and an ideal amount of reactive surface area. These loamy soils tend to drain well, but they can hold sufficient water and nutrients in their pore spaces and on particle surfaces to provide for plant growth and optimal water relations. Soils at the extremes of texture are generally overwhelmed by the individual characteristics of their primary particle size and either do not hold much water 32 Water-Efficient Landscaping in the Intermountain West (sands), resist the infiltration of air and water (clay), or do not hold together well and are easily eroded (silts). Plants use water that is stored in a soil’s pores and adsorbed to soil surfaces...

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